Method for esterifying (meth)acrylic acid with an alkanol

ABSTRACT

In a process for esterifying (meth)acrylic acid with an alkanol in the presence of an esterification catalyst, in which unconverted starting compounds and (meth)acrylic ester to be formed are separated off by distillation and an oxyester-containing bottom product is formed, the bottom product is first separated off and then the oxyesters contained in it are cleaved in the presence of a relatively long-chain alkylbenzenesulfonic acid at elevated temperatures.

The present invention relates to a process for esterifying (meth)acrylicacid with an alkanol in the presence of an esterification catalyst, inwhich unconverted starting compounds and (meth)acrylic ester formed areremoved from the reaction mixture by distillation, leaving behind anoxyester-containing bottom product, the bottom product is separated offand the oxyesters contained therein are thereafter cleaved in thepresence of at least one acid catalyst by the action of elevatedtemperatures.

In this-application, the term “(meth)acrylic acid” is an abbreviationfor acrylic or methacrylic acid.

Furthermore, the term “oligomeric (meth)acrylic acid” used in thefurther course of this application means the Michael adducts of(meth)acrylic acid with itself and with the resulting secondaryproducts. Such Michael adducts may be characterized by the formula(III),

where z is an integer from 1 to 5 and R′ is H or CH₃,

and are to be distinguished here from (monomeric) (meth)acrylic acid andfrom (meth)acrylic acid polymers (which are obtainable by free radicalpolymerization of (meth)acrylic acid). The essential feature is that theMichael addition reaction of (meth)acrylic acid with itself and itsresulting secondary products is reversible. Oligomeric (meth)acrylicacid is obtained, for example, in the distillative treatment of (forexample crude) (meth)acrylic acid (the term “crude” indicates a smallamount of, in particular, aldehydic impurities still present) in thebottom product (cf. for example DE-A 22 35 326).

Usually, the preparation of alkyl esters of (meth)acrylic acid byesterifying (meth)acrylic acid with alkanols at elevated temperature iscarried out in the liquid phase with or without solvents and in thepresence of, as a catalyst, acids other than (meth)acrylic acid (cf. forexample DE-A 23 39 519). The disadvantage of this method of preparationis that, as secondary reactions under the abovementioned esterificationconditions, still unconverted starting alcohol undergoes addition at theethylenically unsaturated double bond of already formed alkyl(meth)acrylate (Michael addition reaction) with formation of a compoundof the formula I below and still unconverted (meth)acrylic acidundergoes addition at said double bond with formation of a compound ofthe formula II.

Successive multiple addition is also possible. Furthermore, mixed typescan occur. These adducts (alkoxyesters and acyloxyesters) are referredto as oxyesters for short:

where x and y are integers from 1 to 5,

R is alkyl and

R′ is R or CH₃.

The problem of oxyester formation is particularly pronounced in thepreparation of esters of acrylic acid, the oxyesters mainly formed beingthe alkoxypropionic esters and the acyloxypropionic esters where x and yare 1. In the preparation of esters of methacrylic acid, the oxyesterformation takes place to a lesser extent. The formation of oxyesters isdescribed, inter alia, in DE-A 23 39 529 and in U.S. Pat. No. 5,734,075.The above publications confirm that the formation of oxyesters takesplace essentially independently of the special esterificationconditions. Of very particular importance is the oxyester formation inthe preparation of acrylic esters of C₁- to C₈-alkanols, in particularof C₄- to C₈-alkanols, very particularly in the preparation of n-butylacrylate and 2-ethylhexyl acrylate (for which reason the presentinvention is used in particular in connection with theseesterifications).

Typical of the oxyesters is that their boiling point is above theboiling points of starting acid, starting alcohol, desired esters formedand any organic solvent present.

The working up of any desired esterification reaction mixture is usuallycarried out by separating unconverted starting compounds and resultingdesired esters from the reaction mixture by distillation, the acidcatalyst used for the esterification being separated off beforehand, ifrequired, by extraction by means of water and/or aqueous alkali (cf. forexample Ullmann's Encyclopedia of Industrial Chemistry, Vol. A1, 5thEd., VCH, page 167 et seq.). The bottom product remaining in suchworking up by distillation contains the oxyesters, which result inconsiderably lower yields.

The prior art (e.g. DE-A 19 701 737, DE-A 19 536 191, DE-A 19 536 184,DE-A 19 547 485, DE-A 19 547 459 and CN-A 1063678) therefore containsvarious processes which cleave the oxyesters contained in the bottomproduct separated off, in the presence of at least one acid catalyst bythe action of elevated temperatures, and separate off the resultingcleavage products, preferably by evaporation. Recommended suitableacidic cleavage catalysts are protic acids other than monomeric andoligomeric (meth)acrylic acid (acid strength>that of (meth)acrylicacid), for example mineral acids, such as sulfuric acid or phosphoricacid, and organic acids, such as methanesulfonic acid orp-toluenesulfonic acid. These definitions also apply to thisapplication.

The disadvantage of the cleavage processes of the prior art is howeverthat the resulting cleavage residue is generally highly viscous and as arule contains solids.

Consequently, the cleavage residue is scarcely pumpable and cantherefore be disposed of only with difficulty. Moreover, the generallytar-like solid is deposited on the wall surfaces in the course of time(fouling), which, for example, reduces the passage of heat or can leadto blockages, making it necessary to clean the wall surfaces from timeto time.

U.S. Pat. No. 5,734,075 recommends carrying out the cleavage process inthe absence of acidic cleavage catalysts and instead in the presence ofoligomeric (meth)acrylic acid to reduce the abovementioned problems.However, the disadvantage of this procedure is that the cleavage, inparticular in the case of bottom products which originate fromesterifications with relatively long-chain alkanols, takes placecomparatively slowly and gives only comparatively low conversions.

DE-A 19 536 184 recommends separating the oxyesters from the bottomproduct by distillation before cleaving in order to reduce saidproblems. The disadvantage of this procedure is the necessity of anadditional distillation step.

The addition of a solvent as a diluent for reducing the viscosityproblems has also been proposed. However, the requirement of anadditional component is likewise disadvantageous.

It is an object of the present invention to provide a process foresterifying (meth)acrylic acid with an alkanol in the presence of anesterification catalyst, in which unconverted starting compounds and(meth)acrylic ester formed are removed from the reaction mixture bydistillation, leaving behind an oxyester-containing bottom product, thebottom product is separated off and the oxyesters contained therein arethereafter cleaved in the presence of at least one acid catalyst by theaction of elevated temperature, which process does not have the stateddisadvantages of the processes of the prior art.

We have found that this object is achieved by a process for esterifying(meth)acrylic acid with an alkanol in the presence of an esterificationcatalyst, in which unconverted starting compounds and (meth)acrylicester formed are removed from the reaction mixture by distillation,leaving behind an oxyester-containing bottom product, the bottom productis separated off and the oxyesters contained therein are thereaftercleaved in the presence of at least one acid catalyst by the action ofelevated temperatures, wherein at least one aromatic sulfonic acid ofthe formula IV,

where

R″ independently of one another are each alkyl of six to twenty carbonatoms,

u is an integer from 1 to 3, and

v is 1 or 2,

is present as the at least one acid catalyst for cleaving the oxyesters.

The abovementioned sulfonic acids of the formula (IV) are disclosed, forexample, in EP-A 521 488. u may be 1 or 2 or 3 and v may be 1 or 2.Frequently, the radicals R″ are alkyl radicals of 8 to 16 or 10 to 14carbon atoms. Suitable typical compounds (IV) are accordingly, forexample, octylbenzenesulfonic acids, such as n-octylbenzenesulfonicacid, nonylbenzenesulfonic acids, such as n-nonylbenzenesulfonic acid,decylbenzenesulfonic acids, such as n-decylbenzenesulfonic acid,undecylbenzenesulfonic acids, such as n-undecylbenzenesulfonic acid,dodecylbenzenesulfonic acids, such as n-dodecylbenzenesulfonic acid,tridecylbenzenesulfonic acids, such as n-tridecylbenzenesulfonic acid,tetradecylbenzenesulfonic acids, such as n-tetradecylbenzenesulfonicacid, pentadecylbenzenesulfonic acids, such asn-pentadecylbenzenesulfonic acid, hexadecylbenzenesulfonic acids, suchas n-hexadecylbenzenesulfonic acid, heptadecylbenzenesulfonic acids,such as n-heptadecylbenzenesulfonic acid, octadecylbenzenesulfonicacids, such as n-octadecylbenzenesulfonic acid,nonyldecylbenzenesulfonic acids, such as n-nonyldecylbenzenesulfonicacid, heptadecylbenzenesulfonic acids, such asn-heptadecylbenzenesulfonic acid, octadecylbenzenesulfonic acids, suchas n-octadecylbenzenesulfonic acid, nonyldecylbenzenesulfonic acids,such as n-nonyldecylbenzenesulfonic acid, and eicosylbenzenesulfonicacids, such as n-eicosylbenzenesulfonic acid. According to theinvention, it is of course also possible to use mixtures of compounds(IV). Such mixtures are used as a rule when compounds (IV) which areonly of technical-grade purity are used. Examples of suchtechnical-grade, commercially available compounds (IV) are thealkylbenzenesulfonic acids Bio-Soft® S-100 (average molecular weightabout 318, average R″ chain length=11.5 carbon atoms;manufacturer=Stepan Co.), AAS-985 (linear alkylbenzenesulfonic acidhaving an average alkyl chain length of C₁₁-C₁₂,manufacturer=Continental Chemical Co.), Vista SA 697 and Vista SA 597(linear alkylbenzenesulfonic acids having an average molecular weight of342 and 318, respectively, manufacturer=Vista Chemical Co.), Stepantan®H-100 (a branched dodecylbenzenesulfonic acid, manufacturer=Stepan Co.)and a technical-grade alkylbenzenesulfonic acid from Alfa Products Co.,in which 1% by weight of R″ comprises C₁₀, 40% by weight comprises C₁₁,28% by weight comprises C₁₂ and 31% by weight comprises C₁₃.

In the novel process, the sulfonic acids (IV) to be used according tothe invention can be employed both as sole acidic cleavage catalysts andas a mixture with the acidic cleavage catalysts recommended in the priorart cited (e.g. sulfuric acid, phosphoric acid, methanesulfonic acidand/or p-toluenesulfonic acid), i.e. the molar amount of the novelcompounds (IV) may be, for example, ≧1 mol %, ≧5 mol %, 10 mol %, ≧15mol %, ≧25 mol %, ≧50 mol %, ≧75 mol %, ≧90 mol %, ≧95 mol % or 100 mol%, based on the total amount of acidic cleavage catalysts used in thenovel process. Preferably, the abovementioned amount of the novelcompounds (IV) is at least 25 mol %, particularly preferably at least 50mol %, very particularly preferably at least 75 mol % and particularlyadvantageously 100 mol %.

As a rule, the novel cleavage is carried out in the presence of a totalamount of from 1 to 50, frequently from 1 to 40 or from 5 to 20, % byweight, based on the amount of the oxyesters to be cleaved, of acidiccleavage catalysts.

Furthermore, the novel cleavage can be carried out according to DE-A 19547 459 or DE-A 19 547 485 in the additional presence of monomericand/or oligomeric (meth)acrylic acid. The amount of such monomericand/or oligomeric (meth)acrylic acid may be up to 50% by weight or more,based on the amount of oxyesters to be cleaved. Frequently, theabovementioned amount of monomeric and/or oligomeric (meth)acrylic acidwill be from 5 to 50 or from 10 to 40 or from 20 to 35% by weight.

Usually, the monomeric and/or oligomeric (meth)acrylic acid are added tothe bottom product to be subjected to the cleavage in a conventionalform stabilized by means of polymerization inhibitors. In a particularlysimple manner, the oligomeric (meth)acrylic acid used may be the bottomproduct which is obtained in the distillative purification of crude(meth)acrylic acid and contains mainly compounds of the formula (III)(cf. for example DE-A 22 35 326). The presence of monomeric and/oroligomeric (meth)acrylic acid in the novel cleavage results inparticular in reduced formation of ether and olefin byproducts.

In addition, according to DE-A 19 701 737, the novel cleavage of thebottom product can be carried out in the presence of water. Theabovementioned amount of water is as a rule from 0.1 to 20, frequentlyfrom 1 to 10, % by weight, based on the amount of the oxyesters to becleaved.

The bottom product to be cleaved and the compounds (IV) to be usedaccording to the invention and any other acidic cleavage catalystslikewise to be added and monomeric and/or oligomeric (meth)acrylic acidand any water can be added to the bottom product to be cleaved beforethe latter is transferred to the cleavage reactor. However, they canalso be fed separately to the cleavage reactor. A part or all of theacidic cleavage catalysts required according to the invention may alsobe the acidic esterification catalysts. According to an advantageousembodiment of the invention, the novel cleavage is carried out in thepresence of molecular oxygen.

It is particularly advantageous if a stripping gas, which preferablycontains molecular oxygen, is passed as an entraining agent for thecleavage products through the mixture to be cleaved in the novelprocess. The stripping gas used is advantageously air or a mixture ofair with inert gas (e.g. nitrogen).

The cleavage temperature to be used according to the invention is as arule from 140 to 260° C., frequently from 180 to 230° C. The novelcleavage is preferably carried out at atmospheric or reduced pressure(<1 bar), typically at from 500 to 700, frequently from 40 to 300, mbar(so that the cleavage products evaporate immediately).

If a stripping gas is passed through the cleavage mixture during thecleavage, its amount is usually 1-100 l per h per l. As a rule, thecleavage requires reaction times of from 1 to 15 hours. The conversionof the cleavage is usually ≧90% by weight.

For example, a simple heatable stirred reactor having double-jacketheating or heating coil or a forced-circulation evaporator, for examplea falling film evaporator or flash evaporator, coupled with a dwelltank, can be used for carrying out the novel cleavage. For betterseparation of the cleavage products from the bottom product, arectification apparatus mounted on the cleavage apparatus, for example apacked or tray column, may be expedient. This rectification apparatus isas a rule operated with stabilization with polymerization inhibitors(e.g. phenothiazine, hydroquinone monomethyl ether, hydroquinone, etc.).Of course, the bottom product to be cleaved and originating from theesterification is also stabilized against polymerization by means ofpolymerization inhibitors.

The reaction in the novel cleavage takes place, for example, by aprocedure in which the bottom product to be cleaved is removedcontinuously from the distillative working-up of the esterificationmixture and is fed, with the cleavage catalyst to be used according tothe invention, any monomeric and/or oligomeric (meth)acrylic acid andany water, to the cleavage reactor. However, the reaction can also becarried out batchwise. Also possible is a semicontinuous reaction inwhich the bottom product to be cleaved and any additives to be added tosaid bottom product are added continuously to the cleavage reactor whichcontains the acidic cleavage catalyst, and the bottom product obtainedin the cleavage is removed batchwise from the cleavage reactor onlyafter the end of the cleavage.

The cleavage products formed in the novel cleavage (alkanol, alkyl(meth)acrylate and (meth)acrylic acid) are usually separated offcontinuously in vapor form and, in contrast to the process of U.S. Pat.No. 5,734,075, can be recycled directly to the esterification withoutintermediate purification. Of course, the recycling could however alsobe carried out according to U.S. Pat. No. 5,734,075.

If the esterification is carried out so that the water formed in theesterification is separated off continuously via a rectification columnmounted on the esterification reactor, the recycling of the cleavageproducts to the esterification is preferably carried out via thisrectification column (recycling is expediently effected into the lowerhalf of the rectification column).

The novel cleavage process can of course also be carried out in aplurality of stages (for example in a cascade, for example as in CN-A1063678).

Preferably, the novel cleavage is carried out in two stages, the contentof acidic cleavage catalysts being brought to, as a rule, from 1 to 20%by weight in the first cleavage stage and from 5 to 40% by weight in thesecond stage, based on the contained amount of oxyesters to be cleaved.The residence time in the individual stages may be identical ordifferent. Preferably, it increases from the first to the last stage. Ina two-stage procedure, the residence time of the cleavage mixture in thefirst stage is expediently from 1 to 15 h and that in the second stageis from 10 to 40 h.

Furthermore, the cleavage temperature in a multistage procedurepreferably increases toward the last stage. In the two-stage case, thecleavage temperature in the first stage is expediently from 160 to 200°C. and that in the second stage is from 180 to 220° C.

The advantage of the stepwise cleavage described above is that thebottom product obtained in the esterification generally still containssignificant amounts of the desired ester which, at the high cleavagetemperatures, is particularly susceptible to (free radical)polymerization and is not inert even to the acidic cleavage catalysts.Under comparatively mild cleavage conditions in the first stage, theseamounts of desired ester may be separated off essentially unchanged withthe resulting cleavage products in a mild manner before the cleavage canbe completed in the subsequent stages under more severe cleavageconditions. When a multistage cleavage is carried out continuously, thepressure in successive stages may be identical or different. In the caseof constant pressure, the transport from one stage to the other may beeffected in a simple manner by level-controlled overflow. At differentpressures, transport of the mixture by means of pumping is advisable.

In a multistage cleavage, it is furthermore advantageous if the residueof the last cleavage stage is at least partly recycled (expediently from10 to 80% of its weight) to the first cleavage stage.

Furthermore, different amounts of monomeric and/or oligomeric(meth)acrylic acid, of water and of acidic cleavage catalysts may be fedto the individual stages.

The applicability of the novel cleavage process is not restricted to thespecial nature of the esterification process, the byproducts of whichare the oxyesters, i.e. the addition compounds I and II. As a rule, theesters are prepared by conventional processes (cf. Ullmann'sEncyclopedia of Industrial Chemistry, Vol. A1, 5th Ed., VCH, page 167 etseq.). Esterification processes catalyzed by means of ion exchangers areof course also suitable.

A typical example of the conditions under which the esterificationpreceding the cleavage of the oxyesters can take place may be describedbriefly as follows:

Alcohol: (meth)acrylic 1:0.7-1.2 (molar) acid Catalyst: Sulfuric acid orsulfonic acids (e.g. p-toluenesulfonic acid) Amount of catalyst: 0.1-10%by weight (preferably 0.5-5% by weight), based on starting materialsStabilization: 200-2000 ppm of phenothiazine (based on the weight of thestarting materials) Reaction temperature: 80-160° C., preferably 90-130°C. Reaction time: 1-10 h, preferably 1-6 h

If required, an entraining agent (e.g. cyclohexane or toluene) is usedfor removing the water of esterification. The esterification may becarried out at atmospheric, superatmospheric or reduced pressure, bothcontinuously and batchwise.

The acrylic acid used for the esterification may have been produced, forexample, by catalytic gas-phase oxidation of propene and/or acrolein orpropane and may contain typically from 0.05 to 0.5% by weight ofaldehydes, from 0.1 to 5% by weight of acetic acid, from 0.05 to 2% byweight of maleic acid/maleic anhydride, from 0.1 to 5% by weight ofwater, from 0.1 to 1% by weight of polymerization inhibitors (e.g.phenolthiazine), from 0.1 to 5% by weight of oligomeric acrylic acid andfrom 80 to 99% by weight of acrylic acid.

In the acid-catalyzed esterification of acrylic acid with alkanols, thebottom product resulting after the removal of the acidic esterificationcatalyst, of the unconverted starting materials and of the acrylic esterhas, as a rule, the following composition:

1-20% by weight of acrylic esters 40-80% by weight of alkoxypropionates(cf. formula I) 5-30% by weight of acyloxypropionates (cf. formula II)Remainder: mainly stabilizers (phenothiazine) and polymers from 0.1 to2% by weight of maleic esters may likewise be present.

Further details and advantages of the novel process are evident from theembodiments described below.

The advantage of the novel procedure is that the residue remaining afterthe cleavage and isolation of the cleavage products is generally ofpumpable consistency and contains at most a minimum solids load.

EXAMPLES AND COMPARATIVE EXAMPLES Example 1

A first circulation reactor consisting of glass (volume 1 l), heated bymeans of a heating cartridge, was filled with 500 g of anoxyester-containing bottom product originating from the preparation ofbutyl acrylate and freed from the acidic esterification catalyst (below,butyl is generally n-butyl). The bottom product contained 15% by weightof butyl acrylate, 60.5% by weight of butoxyester I (R=C₄H₉) and 17% byweight of acyloxyester II (R=C₄H₉). The remainder consisted of (freeradical) polymers, oligomers and polymerization inhibitor(phenothiazine).

60 g of dodecylbenzenesulfonic acid and 95 g of acrylic acid (based onthe weight of the acrylic acid stabilized with 300 ppm of phenothiazine)were then added to the circulation reactor.

180 g of the above bottom product, 40 g of acrylic acid stabilized asabove and 20 g of dodecylbenzenesulfonic acid were fed continuously perhour to the circulation reactor filled in this way, the feed beinglevel-controlled.

The cleavage temperature was 175° C. and the pressure was 700 mbar. Thecleavage products were removed in vapor form, via a packed columnmounted on the circulation reactor (50 cm (packing height)×2.8 cm(internal diameter), 0.8 cm (diameter) Raschig rings) as a splash guardand were condensed. 148 g of condensate were obtained hourly.

The cleavage residue of the first circulation reactor was fed to asecond circulation reactor of identical design, the feed beinglevel-controlled (overflow). In addition, 40 g/h of the abovementionedstabilized acrylic acid were fed to the latter reactor. The cleavagetemperature in the second circulation reactor was 200° C. The pressurewas likewise 700 mbar. The cleavage products of the second circulationreactor were removed as in the case of the first circulation reactor andcondensed. 92.5 g of condensate were obtained hourly. The two condensatestreams obtained were combined and were analyzed by gas chromatography.According to this, the combined condensate contained (based on itsweight):

64.8% by weight of butyl acrylate,  4.2% by weight of butanol,  0.2% byweight of dibutylether and  1.4% by weight of butenes.

The conversion in the cleavage was thus 98% by weight, based on theamount of oxyesters contained in the bottom product. The cleavageresidue (bottom discharge of the 2nd circulation reactor) was pumpableand contained no solid. Even after the cleavage residue had been storedfor 24 hours at 25° C., no solid separated out.

Example 2

A circulation reactor consisting of glass (volume 1 l), heated by meansof a heating cartridge, was filled with 500 g of an oxyester-containingbottom product of Example 1, originating from the preparation of butylacrylate and freed from the acidic esterification catalyst. 60 g ofdodecylbenzenesulfonic acid and 95 g of acrylic acid (based on theweight of the acrylic acid stabilized with 300 ppm of phenothiazine)were then added to the circulation reactor.

95 g of the bottom product of Example 1, originating from thepreparation of butyl acrylate, 20 g of acrylic acid stabilized as aboveand 20 g of dodecylbenzenesulfonic acid were fed continuously per hourto the circulation reactor filled in this way, the feed beinglevel-controlled. The cleavage temperature was 195° C. and the pressurewas 700 mbar. The cleavage products were removed as in Example 1, invapor form, via a packed column mounted on the cleavage reactor (50 cm(packing height) 2.8 cm (internal diameter), 0.8 cm (diameter) Raschigrings) as a splash guard and were condensed. 73 g of condensate wereobtained hourly.

According to gas chromatographic analysis, the condensate contained(based on its weight):

61.3% by weight of butyl acrylate,  4.0% by weight of butanol,  0.3% byweight of dibutylether and  2.1% by weight of butenes.

The conversion in the cleavage was thus 97% by weight, based on theamount of oxyesters contained in the bottom product. The cleavageresidue (bottom discharge) was pumpable and contained no solid. Evenafter the cleavage residue had been stored for 24 hours at 25° C., nosolid separated out.

Comparative Example 1

As in Example 1, except that the dodecylbenzenesulfonic acid wasreplaced by an equimolar amount of p-toluenesulfonic acid.

The combined condensate contained

64.2% by weight of butyl acrylate,  4.5% by weight of butanol,  0.2% byweight of dibutylether and  1.5% by weight of butenes.

The conversion in the cleavage was thus 97% by weight, based on theamount of oxyesters contained in the bottom product. The cleavageresidue was pumpable but contained about 1% by weight of solids, some ofwhich separated out when the cleavage residue was left to stand at 25°C.

Comparative Example 2

In a 1 l stirred reactor, a mixture of 180 g of the bottom product ofExample 1, originating from the preparation of butyl acrylate, and 120 gof oligomeric acrylic acid stabilized with phenothiazine (bottom productof a distillation of crude acrylic acid) was heated at 700 mbar and1950C. The resulting cleavage products were removed via a packed columnmounted on the cleavage reactor (30 cm (packing height)×2.8 cm (internaldiameter), 0.8 cm (diameter) Rashig rings) as a splash guard and werecondensed. 160 g of condensate were obtained in the course of 6.5 hours.

The condensate contained

64.0% by weight of acrylic acid, 26.8% by weight of butyl acrylate, 0.8% by weight of butanol and  6.0% by weight of butylbutoxypropionate.

The conversion in the cleavage was thus <10% by weight, based on theamount of oxyesters contained in the bottom product.

We claim:
 1. A process for esterifying (meth)acrylic acid with analkanol in the presence of an esterification catalyst, in whichunconverted starting compounds and (meth)acrylic ester formed areremoved from the reaction mixture by distillation, leaving behind anoxyester-containing bottom product, the bottom product is separated offand the oxyesters contained therein are thereafter cleaved by the actionof elevated temperature in the presence of at least one acid catalyst,at least one aromatic sulfonic acid of the formula IV,

where R″ independently of one another are each alkyl of six to twentycarbon atoms, u is an integer from 1 to 3, and v is 1 or 2, beingpresent as the at least one acid catalyst for cleaving the oxyesters,wherein the cleavage is carried out stepwise.
 2. A process as claimed inclaim 1, wherein the amount of the aromatic sulfonic acids of theformula (IV) is at least 50 mol %, based on the total amount of the acidcatalysts used for the cleavage of the oxyesters.
 3. A process asclaimed in claim 1, wherein the amount of the aromatic sulfonic acids ofthe formula (IV) is at least 75 mol %, based on the total amount of theacid catalysts used for the cleavage of the oxyesters.
 4. A process asclaimed in claim 1, wherein the amount of the aromatic sulfonic acids ofthe formula (IV) is 100 mol %, based on the total amount of the acidcatalysts used for the cleavage of the oxyesters.
 5. A process asclaimed in claim 1, wherein the total amount of acidic cleavagecatalysts is from 1 to 50% by weight, based on the amount of theoxyesters to be cleaved.
 6. A process as claimed in claim 1, wherein thetotal amount of acidic cleavage catalysts is from 5 to 20% by weight,based on the amount of the oxyesters to be cleaved.
 7. A process asclaimed in claim 1, wherein the cleavage of the oxyesters present in thebottom product is effected in the presence of up to 50% by weight, basedon the amount of the oxyesters to be cleaved, of monomeric and/oroligomeric (meth)acrylic acid.
 8. A process as claimed in claim 1,wherein the cleavage of the oxyesters present in the bottom product iseffected in the presence of from 10 to 40% by weight, based on theamount of the oxyesters to be cleaved, of monomeric and/or oligomeric(meth)acrylic acid.
 9. A process as claimed in claim 1, wherein thecleavage of the oxyesters present in the bottom product is effected inthe presence of from 0.1 to 20% by weight, based on the amount of theoxyesters to be cleaved, of water.
 10. A process as claimed in claim 1,wherein the cleavage of the oxyesters present in the bottom product iseffected in the presence of from 1 to 10% by weight, based on the amountof the oxyesters to be cleaved, of water.
 11. A process as claimed inclaim 1, wherein dodecylbenzenesulfonic acid is present as at least onearomatic sulfonic acid of the formula (IV).
 12. A process as claimed inclaim 1, wherein the cleavage temperature is from 140 to 260° C.
 13. Aprocess as claimed in claim 1, wherein the cleavage temperature is from180 to 230° C.
 14. A process as claimed in claim 1, wherein the cleavageis carried out at from 500 mbar to 1 bar.
 15. A process as claimed inclaim 1, wherein a stripping gas is passed through the bottom product.16. A process as claimed in claim 1, wherein the stripping gas used isan oxygen-containing gas.
 17. A process as claimed in claim 1, whereinthe cleavage products obtained are immediately recycled to theesterification.
 18. A process as claimed in claim 1, wherein the alkanolis a C₁- to C₈- alkanol.
 19. A process as claimed in claim 1, whereinthe alkanol is n-butanol or 2-ethylhexanol.
 20. A process as claimed inclaim 1, wherein the cleavage is carried out in two stages.
 21. Aprocess as claimed in claim 20, wherein the content of acidic cleavagecatalysts is from 1 to 20% by weight in the first cleavage stage andfrom 5 to 40% by weight in the second stage, based on the containedamount of oxyesters to be cleaved.
 22. A process as claimed in claim 20,wherein the cleavage temperature in the first stage is from 160 to 200°C. and that in the second stage is from 180 to 220° C.
 23. A process asclaimed in claim 20, wherein the residence time in the first cleavagestage is from 1 to 15 h and that in the second stage is from 10 to 40 h.24. A process as claimed in claim 1, wherein the residue of the lastcleavage stage is at least partly recycled to the first cleavage stage.